The present disclosure relates generally to an electrochemical cell and more particularly to an electrochemical cell constructed to improved serviceability and scalability attributes.
Electrochemical cells such as fuel cells convert a fuel into usable energy via electrochemical reactions rather than by combustion. As such—and in addition to having fewer failure-prone mechanical parts—electrochemical cells have several environmental advantages over internal combustions engines (ICEs) and related power-generating sources. In one common form, when configured as a fuel cell—such as a proton exchange membrane or polymer ion-exchange membrane (in either event, PEM) fuel cell—defines a generally thin two dimensional structure that includes a pair of generally planar catalyzed electrodes that are separated by a generally planar ion-transmissive medium (such as those made with a perfluorosulfonic acid, a commercial version of which is Nafion™) in what is commonly referred to as a membrane electrode assembly (MEA). Generally planar plates with serpentine reactant flow channels formed on one or both opposing major surfaces are disposed against the electrodes; in this way, the electrochemical reaction occurs when a first reactant in the form of a gaseous reducing agent (such as hydrogen, H2) is introduced through the channels to be ionized at the anode and then made to pass through the porous electrodes and ion-transmissive medium such that it combines with a second reactant in the form of a gaseous oxidizing agent (such as oxygen, O2) that has been introduced through the channel of another plate that is placed facingly adjacent the other electrode (the cathode); this combination of reactants form water as a byproduct. The electrons that were liberated in the ionization of the hydrogen proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load (such as an electric motor) where useful work may be performed. The power generation produced by this flow of DC electricity is typically increased by combining numerous such cells into a larger current-producing assembly. In one such construction, the fuel cells are connected in series along a common stacking dimension in the assembly—much like a deck of cards—to form a fuel cell stack. Depending on the power output required, such stacks may include a large number (often between about two hundred and three hundred) of individual stacked cells.
Similarly, in flow batteries, stored chemical energy is converted to power during battery discharging, while chemical species are electrochemically produced and stored during charging. Tanks are used to store the electrolyte solutions, namely, the anolyte and the catholyte that are pumped in order to circulate between the tanks and the electrochemical cell. During discharging of the battery, the anolyte is pumped between its tank and the electrochemical cell anode where it undergoes electrochemical oxidation, while the catholyte is pumped between its tank and the cathode where it undergoes electrochemical reduction. The two electrodes of the cell are separated by an ion exchange membrane that allows the selective diffusion of specific ionic species, like protons or hydroxyl ions. As in the case of the fuel cell, the stack in the flow battery includes multiple elemental units (that is to say, the electrochemical cells) electrically connected in series (for example, through bipolar plates) and hydraulically connected in parallel to form a stack of cells.
Such a stack of fuel cells or flow batteries is ordinarily assembled under compression in order to seal the cells or batteries and to secure and maintain a low interfacial electrical contact resistance between the reactant plates, the gas diffusion media, and the catalyst electrodes. A desired compression load on the fuel cell stack typically ranges from about fifty to about two hundred psi (and sometimes more), and is maintained by a compression retention enclosure housing the fuel cell stack. In one common form, the enclosure includes tie rods that extend through or around the end plates in order to maintain the cells in the compressed state.
A problem with this approach is that the power output of the as-assembled stack is fixed by the number of cells within the stack and the relatively precise, ordered way these cells are compressed and retained. Thus, a use-versus-supply mismatch arises if there is a need for an incremental increase in power that is greater than a single stack can provide, yet far less than that provided by the inclusion of an additional stack; such a situation leads to an inefficient use of the available power. Moreover, should one of the cells within the stack require repair or replacement, disassembly becomes a cumbersome process, as the various structural connections that span the substantial entirety of the stack must be carefully removed in order to ensure that the relatively thin-profiled cells aren't damaged during such disassembly and individual cell removal. Such serviceability concerns are especially prevalent with the thin bipolar plate structures that are used to convey the reactant to the respective electrode assemblies within the cell.
Additional sealing may be achieved through the use of numerous gaskets or related seals, often including at least one seal for each exposed surface of the reactant plates, as well as at other locations in and around the MEA. This too presents problems in that the large number of seals makes cell assembly more difficult and expensive. Furthermore, the use of the compression loads placed on the assembled stack has a tendency to cause these seals or gaskets to become either misaligned or overly compressed, either of which can compromise seal integrity.